A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted on various magnetic recording devices, in particular to a perpendicular magnetic recording medium mounted on a hard disk drive used for an external storage device of computers, AV equipment, and other apparatus.
B. Description of the Related Art
The recording density of hard disk drives (HDDs) is rapidly increasing and this trend is likely to continue in the future. As a result of this remarkable enhancement of the recording density, “thermal fluctuation” has become a problem. The thermal fluctuation phenomenon causes failure of stable retention of recorded signals. In the longitudinal magnetic recording system commonly employed now, the higher recording density causes more significant thermal fluctuation. To cope with this problem, a perpendicular magnetic recording system is being actively developed in which the stability of a bit increases with increase of the recording density, which is contrary to the situation with an longitudinal magnetic recording system.
To advance the enhancement of recording density in a magnetic recording medium, the magnetic isolation between the crystal grains composing a magnetic recording layer must be promoted and the magnetization reversal unit needs to be reduced. The thermal stability (resistance to thermal fluctuation) of a magnetic material is represented by an index KuVa, a product of a uniaxial anisotropy constant Ku and an activation volume Va, the latter being known to correlate to the volume of a magnetization reversal unit V. That is, the thermal stability of a magnetic recording medium deteriorates as the KuVa (or KuV) decreases. As is apparent from the index, thermal stability deteriorates with the decrease of the magnetization reversal unit that is required to enhance the recording density. Therefore, a perpendicular magnetic recording medium still raises a problem of thermal fluctuation. So, the Ku must be increased to keep thermal stability even when the magnetization reversal unit is decreased.
Meanwhile, the magnetic field intensity required for recording in an HDD is known to be approximately proportional to the Ku. As a result, the increase of the Ku to keep thermal stability elevates the magnetic field intensity required for recording. If the increase of the magnetic field intensity is significant, recording may become impossible.
With a decrease in the magnetization reversal unit, the demagnetizing field decreases, and the switching field of a magnetic recording layer increases. Consequently, the magnetic field intensity required for recording increases when the magnetization reversal unit is decreased.
The miniaturization of the magnetization reversal unit and the increase of Ku, both directed to high recording density, contribute to improve the recording resolution and the thermal stability of a magnetic recording medium. However, both of these tend to lower the easiness with which a magnetic recording medium is recorded, referred to as “recordability.”
Thus, a method is demanded that improves the thermal stability and the read-write performance of a magnetic recording medium without lowering the recordability. To solve this problem, a method has been proposed in which the magnetic layer is divided into two or more layers, and the layers are deposited with varied compositions or a nonmagnetic layer(s) is inserted between the divided magnetic layers. (See for example, Japanese Unexamined Patent Application Publication No. 2003-157516.) This document discloses that by interrupting the epitaxial growth to divide a magnetic recording layer, media noise of the magnetic recording medium can be reduced while preserving a volume of magnetization reversal in it to improve thermal stability. However, the distance between the recording head and the soft magnetic backing layer increases in a thick magnetic recording layer, and the switching field increases when the magnetization reversal unit is decreased directing to high density recording. Therefore, the recordability inevitably deteriorates in the magnetic recording medium in this method.
Japanese Unexamined Patent Application Publication No. H2004-39033 proposes a method in which the reverse magnetic domain noise is reduced and the resistance to thermal fluctuation is improved by applying antiferromagnetic coupling (AFC) employed in longitudinal magnetic recording media to a perpendicular magnetic recording medium. It has been disclosed, however, that the use of antiferromagnetically coupled magnetic recording layers causes an increase of the switching field. (See for example, Erol Girt et al., IEEE Trans. on Magn. (United States of America), Vol. 39, No. 5, p. 2306-2310 (2004).) Thus, degradation of the recordability is essentially inevitable.
The decrease of a magnetization reversal unit improves noise and S/N (signal to noise ratio) of a magnetic recording medium. The decrease of a magnetization reversal unit, however, degrades thermal stability as described above, and also increases the switching field of the magnetic recording medium due to a decrease of the demagnetizing magnetic field. The increase of the Ku and a thickness of the magnetic recording layer for preserving thermal stability also tend to increase the switching field of a magnetic recording medium. Although the reduction of the magnetization reversal unit is essential to achieve high recording density of a magnetic recording medium, on the one hand, this is in a trade-off relationship with the recordability of the magnetic recording medium. This circumstance has been an obstacle to advancing high density recording of a magnetic recording medium.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.